Abstract: The present invention provides a thin-walled PTFE tube with appropriate elongation, sufficient strength, and good uniformity during stretching and a medical tube with excellent flexibility and dimensional accuracy. The polytetrafluoroethylene tube according to the present invention is a polytetrafluoroethylene tube having a wall thickness of about 0.04 mm or less, wherein the wetting tension of either of or both outer and inner surfaces of the polytetrafluoroethylene tube is 46 mN/m or more, and in a stress-strain curve obtained by a tensile test performed under an atmosphere of 200° C., tensile stress at 20% strain ?20 (N/mm2) of the polytetrafluoroethylene tube and tensile stress at 50% strain ?50 (N/mm2) of the polytetrafluoroethylene tube satisfy 2.0?0.1×?20+0.3×?50<5.5 (Inequality (1)).

Abstract: In a method for growing ribbon-like crystal of crystalline substance, a seed crystal is contacted on the upper surface of melt of crystalline substance raised up to the level beyond the upper edge of a crucible, thereafter the contact interface between said seed crystal or a successively growing crystal and the melt (the solid-liquid interface) is cooled under freezing point of said crystalline substance, thereby the crystal is made to successively grow on said solid-liquid interface, while said seed crystal or said successively growing crystal is drawn out substantially in horizontal direction, wherein the circulating convection is made to cause in such a manner that high temperature melt flows underside of said solid-liquid interface, thereby the crystal growth in the direction of depth of the melt is prevented.

Abstract: A method for growing a thin and flat ribbon-like crystal of crystalline substance, wherein a seed crystal is contacted with horizontal free surface of melt of crystalline substance raised up to the level beyond the upper edge of a crucible, thereafter the contact interface between said seed crystal or a successively growing crystal and the melt (the solid-liquid interface) is cooled under freezing point of said crystalline substance, thereby the crystal is made to successively grow on said solid-liquid interface, while said seed crystal and said successively growing crystal is drawn out substantially in horizontal direction, wherein an outwardly projecting eaves-structure is formed on at least a part of an upper peripheral wall of said crucible, said melt of crystalline substance is guided to said eaves-structure, which is then heated, thereby the generation of recrystallization on upper face of the melt contacting with the upper portion of the peripheral wall of the crucible is prevented, and high cooling ef